Gate valves are typically used when a straight-line flow of fluid and minimum flow restriction are required. Typically, the gate has body with a cavity and a flow passage extending through the body and intersecting the cavity to allow flow through the valve. When the valve is wide open, the gate is drawn into an end of the valve cavity away from the flow passage. The flow passage is typically the same size as the pipe in which the valve is installed.
A typical gate valve used in connection with oil and gas production has a flow passage that intersects a central cavity in the valve. Seats are placed in counterbores formed in the flow passage at the intersection of the flow passage with the cavity. An obstruction or gate is moved past the seats between open and closed positions to seal the cavity from the passage.
The seats generally have seals which seal the seat to the counterbore of the flow passage. These seals prevent the entry of fluid from the central cavity or chamber of the body to the downstream flow passage. When the gate is opened, the seals perform no function. For gate valves designed with unidirectional sealing when the gate is closed, fluid will flow past the upstream seat into the chamber or cavity of the body. The fluid pressure in the chamber is sealed by the seal of the downstream seat formed between the gate and the seat. In addition, a sand screen may also be positioned in the seats to protect the valve from sand intrusion. For gate valves designed with bidirectional sealing when the gate is closed, fluid is maintained on one side of the gate and not allowed to flow into the chamber or cavity of the body.
When gate valves are subjected to high pressure environments, the bearing stress between the seat and the gate increases and thus gate valves must be able to tolerate the increased bearing stress. This is especially the case on the downstream side of the gate as the gate is forced onto the seat. This can often result in local deformation of the gate where it contacts the seat, making it more difficult or impossible to actuate the gate between open and closed positions. To counter this problem, the size of the valve, the actuators, and the tree may be increased, resulting in additional weight and expense.
One approach used to address this issue has been to coat the (metal) gate and seat with a thin coating composed of Tungsten Carbide particles with CoCr metal binder (WCCoCr coating) due to its hardness and low friction. Diamond-like Carbon (DLC) coatings have also been used on top of the WCCoCr coatings to lower the friction further. However, there is a limit to the bearing stress beyond which these coatings tend to locally deform or gall, creating stresses in the coatings and thus limiting bearing stress. Eventually leakage and difficulty in operating the gate valve may result. Another approach to control this bearing stress is to increase the surface area of the seat by increasing its diameter. However this also has the unintended effect of increasing the force applied due to the increased effective pressure-area. The return on controlling bearing stress with this method diminishes with higher pressure valves, resulting in even larger valves and the actuators needed to close and open them. Another approach has been to attempt solid ceramic or cemented carbide gates and seats to achieve a higher bearing stress capacity. The problem thus far is that the high temperatures required for sintering solid parts typically results in degraded carbide properties, which in turn result in a poor surface texture and high friction.
A need exists for a technique to increase bearing stress capacity in gate valves and minimize friction without an increase in the actuator size or production tree.